Smith-Vaniz patent No. 3,542,472 discloses a displacement measuring apparatus having two or more frequencies simultaneously occurring. Tur et al patent No. 4,768,880 describes a device which utilizes a multi-mode laser diode with a low coherence that is substantially shorter than the length being measured. Any suggestions of intensity modulation in either of these patents is incidental inasmuch as it is generally the result of phase modulation. It would appear that the device shown in the Tur et al patent must overcome the same inherent disadvantages of any system using current laser diodes. The Smith-Vaniz patent shows the relationship between intensity and displacement.
Menderin et al patent No. 3,970,389 is concerned with a displacement measuring apparatus that varies laser frequencies to determine an unknown distance. A photodetector is used to monitor the intensity of a phase shifted monochromatic light source. The shifts in both phase and frequency allow for the resolution of ambiguity resulting from the unknown initial position.
Bourdet et al patent No. 4,492,464 is concerned with a displacement measuring apparatus which utilizes dual frequencies from dual source lasers. While this patent shows the use of dual frequencies, their use is for the resolution of ambiguities. This device appears to be an improvement over the apparatus shown in the Menderin et al patent.